Fusarium Wilt of Watermelon in Australia: Biology and Management

Fusarium wilt of watermelon is one of the most economically significant diseases worldwide, causing severe losses. The main pathogen is Fusarium oxysporum f.sp. niveum (Fon) and is present in major watermelon production regions of the world. Currently, there is a severe lack of information regarding the distribution and biology of this important pathogen in Australia. An ad hoc survey of Australian watermelon production regions was conducted to evaluate impact and incidence of wilt disease, and determine if Fon was the predominant pathogen. Incidence and severity ranged from moderate to severe at all affected sites. In total, 749 isolates of Fusarium oxysporum and 335 isolates of Fusarium solani were isolated from plant material. Pathogenicity trials using recovered Fusarium isolates confirmed Fon is the main pathogen of watermelon wilt in Australia. Race differentiation showed that Australian Fon isolates were either race 2 or race 3. This is the first known report of identified Fon races in Australia. Cross pathogenicity trials using Australian Fon race 2 and 3 on other cucurbit hosts, indicate Australian Fon was able to colonize all hosts and cause disease on rockmelons, honeydew and jam melons. Phylogenetic analyses conducted on race typed Australian Fon isolates to explore genetic diversity and possible origin of Fon in Australia, show that Australian Fon is not monophyletic and there were three potential introduction events into Australia. The results indicate that pathogenicity to watermelon evolved independently in these isolates. PCR screening in Australian Fon resulted in the identification of SIX4b, SIX6, SIX9, SIX11, two homologs of SIX13 and a highly divergent SIX8 like gene. Analysis showed there is strong evidence of horizontal gene transfer (HGT) between isolates of Australian Fon, International isolates and isolates in different Clades of the species phylogeny